The present invention relates to a method for controlling the furnace temperature of a multi-zone heating furnace for heating metallic steel slabs or the like.
So far the setting on a furnace temperature control device has been so far made in general by the manual setting by an operator, and as a set point the value which was obtained from the experiences in operations of furnaces has been used. When the furnace temperature is set, firstly it is required that the average slab temperature is within a desired range when a slab is discharged, and secondly the slab must be heated in such a way that the temperature difference between the surfaces and interior of the slab may become minimum. The means which most simply satisfies these demands is considered to heat the slab as rapidly as possible in the initial stage of heating and thereafter soaking the slab at a substantially constant temperature. With the prior art manual operations, it has been difficult to switch finely the furnace temperature set points in response to the conditions of rolling lines so that there has been a tendency for using the simple furnace temperature setting of the type described above. However, this method has a defect that the furnace temperature set point becomes higher as a whole, thus resulting in the increase in a fuel unit.
Recently in view of energy savings, there has been proposed a method for using a computer for determining an optimum furnace temperature set point in response to the distribution of slabs in the furnace and automatically setting this set point. As a result, it has become possible to use the relationship between the furnace temperature and the slab temperatures as a heating model, thereby predicting the heating conditions of slabs. Thus the accuracy with which the slab discharge temperature is controlled has been improved.
For instance, Japanese Patent Application Kokai No. 52-117818 proposes a method wherein the furnace temperature in each heating zone is assumed, the loss of heat of the furnace and the deviation of a predicted discharge temperature of a slab from a set discharge temperature at said assumed furnace temperature are obtained, and the furnace temperature in each heating zone is so determined and set that the sum of the product of said thermal loss and a first coefficient and the product of the square of said deviation and a second coefficient may become minimum.
The prior art methods described above are common in that the relationship between the furnace temperature and slab temperature is used. However, the relationship between the furnace temperature and fuel flow rate has not been directly considered. No consideration has been given to whether the furnace temperature pattern which is obtained can minimize the fuel. Meanwhile, a rolling line for rolling heated metallic steel workpieces or slabs is forced frequently to suspend its operations due to the exchanges of rolling rolls, operation troubles or the like. In this case, when the normal operations continue in the heating furnace, there is a fear that overheating of slabs results in loss of thermal energy. Therefore countermeasures are needed such as lowering the furnace temperature for some period until the resumption of the operations of the rolling line, thereby preventing the slabs from being overheated and contemplating the savings of fuel. Furthermore it is needed to guarantee accuracy of the slab discharge temperature after the resumption of the operations of the rolling line. In one example of the prior art furnace temperature controls in the case of the suspension of the operations of the rolling line, the furnace temperature set point is lowered in such a way that the temperatures of slabs close to the discharge outlet or exit of the furnace may be maintained constant, and prior to the resumption of the discharge of slabs, the furnace temperature is recovered to that when the slab discharge was suspended. With this method, the temperatures of the slabs spaced apart from the outlet of the furnace are deviated from the set points when the slab discharge is resumed, and it takes a time before the furnace temperature control returns to the steady state, resulting in the decrease in productivity. In addition, it is not necessarily true that all conventional heating furnaces have a plurality of zones in which temperatures can be controlled independently of each other. Since temperature control in each zone is not effected, the furnace temperature control in case of the suspension of the operations of the rolling line is not satisfactory.